1. Field of the Invention
The present invention generally relates to run-length limited codes or any finite memory code. The invention is an efficient method for correcting synchronization problems in the recovery of data stored on magnetic or optical media.
2. Background Description
Run-length limited codes are used in many magnetic and optical recording applications. A run-length limited code converts an unconstrained string of data bits into a run-length constrained string which can he more efficiently stored or transmitted. A run-length limited code has a rate which is expressed as a fraction, p/q. This means the encoder converts blocks of p unconstrained bits into blocks of q constrained bits. After passing through the channel, the decoder recovers the blocks of p data bits from the blocks of q constrained bits.
The recovery of data stored on magnetic media is subject to two kinds of errors: incorrectly read bits and loss of synchronization. Incorrectly read bits are corrected by error correcting schemes such as the Reed-Solomon code in which data is passed through an error correcting encoder before channel encoding and decoded after channel decoding. However, if the string of bits entering the channel and the string of bits emerging from the channel are not synchronized, the decoder will be looking across q blocks of constrained bits and will decode the string incorrectly. All subsequently read data is entirely corrupted and will totally defeat the error correcting decoder. The problem is to find an efficient method to correct synchronization problems.